Answer:
A. When oxygen levels within a cell are too low for aerobic respiration
Explanation:
Fermentation is a chemical or metabolic process that occurs in organism, when a carbohydrate such as glucose is metabolized or converted to an alcohol anaerobically (absence of oxygen).
When an organism lacks oxygen within the cell, aerobic respiration would be halted, hence pyruvate produced from glucose further undergoes fermentation anaerobically.
Organisms like lactic acid bacteria and yeast carry out fermentation. The process of fermentation is widely used in food industries, such as in brewing of wine or manufacture of yogurt.